In recent years, a technique for forming a thin film transistor (TFT) by using a semiconductor thin film (having a thickness of approximately several nanometers to several hundred nanometers) formed over a substrate having an insulating surface has attracted attention. Thin film transistors are applied to a wide range of electronic devices such as ICs or electro-optical devices, and prompt development of thin film transistors that are to be used as switching elements in image display devices, in particular, is being pushed. Indium oxide is an example of metal oxides and is used as a light-transmitting electrode material which is necessary for liquid crystal displays and the like.
Some metal oxides have semiconductor characteristics. For example, metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like. Thin film transistors in which a channel formation region is formed using such a metal oxide having semiconductor characteristics are already known (Patent Documents 1 to 4 and Non-Patent Document 1).
Further, not only single-component oxides but also multi-component oxides are known as metal oxides. For example, a homologous compound, InGaO3(ZnO)m (m is natural number) is known as a multi-component oxide semiconductor containing In, Ga, and Zn (also referred to as an In—Ga—Zn—O-based oxide) (Non-Patent Documents 2 to 4).
Furthermore, it is confirmed that an oxide semiconductor containing such an In—Ga—Zn—O-based oxide is applicable to a channel layer of a thin film transistor (Patent Document 5 and Non-Patent Documents 5 and 6).